The present invention relates generally to a grizzly bar device that reduces the size of large lime stones inside a lime kiln. Grizzly bar devices are generally an annular array of teeth-like bars at the lower end of a lime kiln. The bars allow lime pebbles to pass out of the kiln, while retaining large lime stones in the kiln until the large stones are broken into smaller stones. The individual bars of the novel grizzly bar device disclosed herein have tapered surfaces on their flanks (sides) that reduce the tendency of large lime stones becoming caught between the bars, and a tapered front surface to assist in cutting the stones into lime pebbles.
As shown in FIG. 1, a lime kiln 10 is typically a long, hollow combustion chamber cylinder. The diameter of the kiln cylinder is typically 8 feet to 15 feet, and the kiln length is typically 150 feet to 450 feet. The cylinder is conventionally formed of a steel shell having a thickness of three-quarters (xc2xe) of an inch to three and one-half (3.5) inches (2 cm to 9 cm). The kiln cylinder has a slight slope 12, of 1 to 3 degrees for example. The cylinder rotates slowly at, for example, one revolution per minute (RPM). A stationary burner 14 projects a flame into the interior of the kiln to heat the raw material 16.
Raw material 16, such as calcium carbonate (CaCO3) or lime mud, is fed into the upper end 18 of the kiln. The slow rotation of the kiln and its slope 12 cause the raw material to tumble from the inlet, through the kiln, towards the lower kiln end 20. The flame from the burner 14 causes a chemical reaction that converts the raw material 16, e.g., calcium carbonate or lime mud, into calcium oxide (lime) and gaseous carbon dioxide. The lime generated in the kiln by the chemical reaction is typically in pebble form. However, the lime tends to be sticky at the elevated temperatures of 1500xc2x0 F. to 1800xc2x0 F. (815xc2x0 to 982xc2x0 C.) in the kiln. Accordingly, the lime pebbles and lime dust in the kiln can form into larger lime stones.
Large lime stones are broken down to smaller diameter stones and pebbles by the tumbling of the stones in the kiln. The stones are preferably broken down to a nominal size of 5 to 7 inches (12 to 18 cm) or less before the lime is conveyed from the lime kiln to a typical secondary power lime lump crusher. The crusher will further reduce the nominal size of the lime stones to a reduced size, such as 5 inches in diameter or less. The smaller diameter pebbles move from the kiln into and through a floor grate 22, having a typical 10 inch (25 cm) square pattern. The floor grate is adjacent the outlet of the kiln.
In kilns without a grizzly bar device, lime stones fall onto and get stuck on the floor grate 22. The large stones are broken up manually before falling through the grate 22 and into the lump crusher. Human workers stand at the end of a kiln to break lime stones stuck in the grate 22. The workers are in danger of being burned from the radiant heat of the flame of the burner 14, burned from an accidental back draft of the flame, and of being hit by falling lime stones lifted and dropped by the conventional grizzly bars 24 on the rotating kiln. For sake of safety, it is preferable that large stones not become caught in the grate 22. There is also a desire to reduce the need to dedicate operator resources to unplugging the grate 22.
A conventional grizzly bar device 24 breaks large lime stones into smaller stones, before the stones are discharged from the kiln. A grizzly bar device is generally positioned at the end 20 of a kiln to prevent large lime stones from being discharged from the kiln. In a conventional grizzly bar device 24, the bars are equally thick and extend radially inward into the kiln. The gap between adjacent bars decreases from the base to the tip of the bars, because the bars are aligned along radial lines that converge on the axis of the kiln. The converging gap between the bars prevent large lime stones from exiting the lime kiln without first being broken up by the tumbling action of the rotating kiln. Conventional bars xe2x80x9cpinchxe2x80x9d lime stones having a size about the same as the gap between the equally thick bars. A stone becomes xe2x80x9cpinchedxe2x80x9d by sliding partially between two bars and becoming stuck in the grizzly bar device.
As the conventional grizzly bars rotated with the kiln, the pinched lime stones were lifted upwards within the kiln. The pinched stones often fell as they were lifted over the burner tube 26 that extends into the end of the lime kiln and along the centerline 28 of the kiln. These relatively-heavy lime stones have been known to fall onto and damage the burner tube (which typically is operating at a very hot 1800xc2x0 F.). When they fall, the large lime stones have also caused personal injury to workers near the kiln.
Pinched stones, that did not fall when lifted over the burner, have been known to eventually plug all the bars of the grizzly bar device. The pinched stones remain stuck between the bars until they are manually removed or until they fall out from between the bars, which are at the top of the kiln. Conventional bars of a grizzly bar device also allow hot lime dust to remain stationary on the flank of the bars. With time, the accumulation of pinched stones and dust on the bars can block the entire circumference of the grizzly bar device.
A completely plugged grizzly bar device creates a dam at the end 20 of the kiln that prevents the gravity flow of lime out of the kiln. The dam formed by the grizzly device allows the lime product to flood the downhill end of the kiln and allows large stones to flow over the radial bars and out the kiln. These large stones plug the floor grate 22 below the kiln discharge and can excessively allow large stones to enter and damage the lump crusher device. Accordingly, there is a long felt need for a grizzly bar device that does not pinch lime stones and does not become clogged.
A grizzly bar device of the present invention has a double taper on the flanks of its individual bars to prevent stones greater than a nominal diameter, such as five to seven inches in diameter, from exiting the lower end of the kiln. The double taper of the flank includes a first taper parallel to the kiln axis, which causes the bar to become narrower from front to back of the bars. The first (lateral) taper on the flanks causes the gap between bars to become wider from the front to back of the bars. The second (radial) taper on the bar flanks is near parallel to a radial line from the centerline 28 of the kiln and reduces the bar thickness from base to tip of the bars. The second taper on the flanks causes the gap between bars to increase in width from the base to the tip of the bars. The double tapered flanks of the bars prevents the pinching of stones by increasing the gap between bars in two directions. An optional third taper on the front face of individual bars forms a cutting edge on each bar to assist in breaking up large lime stones that roll against the grizzly bar device.
In one embodiment, the invention is a grizzly bar device for a rotating kiln comprising: an annular array of bars having at least two adjacent bars, wherein a thickness of each adjacent bar is reduced from the base to the tip of the bar wherein a gap between said adjacent bars increases in width from the base to the tip of the bars and from the front to the back of the bars. The first (lateral) taper on bar flanks is at an angle parallel to the kiln axis and may be in a range of 3xc2x0 to 15xc2x0. The second (radial) taper on the bar flanks is at an angle to a radial line, and may be in a range of 5xc2x0 to 15xc2x0. Moreover, the front face of each bar may have a cutting corner formed by front taper on the front face that extends from one flank to an opposite flank of an individual bar. The front taper on the front face may be at an angle in a range of 5xc2x0 to 15xc2x0.
In another embodiment, the invention is an annular array of grizzly bars for a rotating kiln comprising: at least two adjacent bars, each bar having a front face and a rear face, wherein the thickness of each bar is reduced from the front to the rear face and also from the base to the tip of the bar, and each of said bars further including a tapered front face, wherein said front taper forms a cutting corner on said front face which is forward in said kiln with respect to an opposite corner of the front face. A gap between the adjacent bars increases in width from the front to the rear of the bars and as the bars extend radially inward of the kiln. The gap is formed by a lateral and radial taper on a flanks of the adjacent bars. The gap allows material that enters the bars to fall through the bars because the gap increases as the lime stones fall by gravity and rotation. A cutting taper on the front face of the bars is rotation sensitive, as the cutting action only works one way. The cutting taper is defined as a taper on the front of a bar, at the face deepest inside the kiln, such that there is a taper line away from a circumferential line from the centerline of the kiln. The cutting taper extends around the tip of the bar so that the top of the bar is also tapered.